Laser-driven light source

ABSTRACT

An apparatus for producing light includes a chamber and an ignition source that ionizes a gas within the chamber. The apparatus also includes at least one laser that provides energy to the ionized gas within the chamber to produce a high brightness light. The laser can provide a substantially continuous amount of energy to the ionized gas to generate a substantially continuous high brightness light.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 11/695,348,filed on Apr. 2, 2007, which is a continuation-in-part of U.S. Ser. No.11/395,523, filed on Mar. 31, 2006, the entire disclosures of which areincorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to methods and apparatus for providing alaser-driven light source.

BACKGROUND OF THE INVENTION

High brightness light sources can be used in a variety of applications.For example, a high brightness light source can be used for inspection,testing or measuring properties associated with semiconductor wafers ormaterials used in the fabrication of wafers (e.g., reticles andphotomasks). The electromagnetic energy produced by high brightnesslight sources can, alternatively, be used as a source of illumination ina lithography system used in the fabrication of wafers, a microscopysystem, or a photoresist curing system. The parameters (e.g.,wavelength, power level and brightness) of the light vary depending uponthe application.

The state of the art in, for example, wafer inspection systems involvesthe use of xenon or mercury arc lamps to produce light. The arc lampsinclude an anode and cathode that are used to excite xenon or mercurygas located in a chamber of the lamp. An electrical discharge isgenerated between the anode and cathode to provide power to the excited(e.g., ionized) gas to sustain the light emitted by the ionized gasduring operation of the light source. During operation, the anode andcathode become very hot due to electrical discharge delivered to theionized gas located between the anode and cathode. As a result, theanode and/or cathode are prone to wear and may emit particles that cancontaminate the light source or result in failure of the light source.Also, these arc lamps do not provide sufficient brightness for someapplications, especially in the ultraviolet spectrum. Further, theposition of the arc can be unstable in these lamps.

Accordingly, a need therefore exists for improved high brightness lightsources. A need also exists for improved high brightness light sourcesthat do not rely on an electrical discharge to maintain a plasma thatgenerates a high brightness light.

The properties of light produced by many light sources (e.g., arc lamps,microwave lamps) are affected when the light passes through a wall of,for example, a chamber that includes the location from which the lightis emitted.

Accordingly, a need therefore exists for an improved light source whoseemitted light is not significantly affected when the light passesthrough a wall of a chamber that includes the location from which thelight is emitted.

SUMMARY OF THE INVENTION

The present invention features a light source for generating a highbrightness light.

The invention, in one aspect, features a light source having a chamber.The light source also includes an ignition source for ionizing a gaswithin the chamber. The light source also includes at least one laserfor providing energy to the ionized gas within the chamber to produce ahigh brightness light.

In some embodiments, the at least one laser is a plurality of lasersdirected at a region from which the high brightness light originates. Insome embodiments, the light source also includes at least one opticalelement for modifying a property of the laser energy provided to theionized gas. The optical element can be, for example, a lens (e.g., anaplanatic lens, an achromatic lens, a single element lens, and a fresnellens) or mirror (e.g., a coated mirror, a dielectric coated mirror, anarrow band mirror, and an ultraviolet transparent infrared reflectingmirror). In some embodiments, the optical element is one or more fiberoptic elements for directing the laser energy to the gas.

The chamber can include an ultraviolet transparent region. The chamberor a window in the chamber can include a material selected from thegroup consisting of quartz, Suprasil® quartz (Heraeus Quartz America,LLC, Buford, Ga.), sapphire, MgF₂, diamond, and CaF₂. In someembodiments, the chamber is a sealed chamber. In some embodiments, thechamber is capable of being actively pumped. In some embodiments, thechamber includes a dielectric material (e.g., quartz). The chamber canbe, for example, a glass bulb. In some embodiments, the chamber is anultraviolet transparent dielectric chamber.

The gas can be one or more of a noble gas, Xe, Ar, Ne, Kr, He, D₂, H₂,O₂, F₂, a metal halide, a halogen, Hg, Cd, Zn, Sn, Ga, Fe, Li, Na, anexcimer forming gas, air, a vapor, a metal oxide, an aerosol, a flowingmedia, or a recycled media. The gas can be produced by a pulsed laserbeam that impacts a target (e.g., a solid or liquid) in the chamber. Thetarget can be a pool or film of metal. In some embodiments, the targetis capable of moving. For example, the target may be a liquid that isdirected to a region from which the high brightness light originates.

In some embodiments, the at least one laser is multiple diode laserscoupled into a fiber optic element. In some embodiments, the at leastone laser includes a pulse or continuous wave laser. In someembodiments, the at least one laser is an IR laser, a diode laser, afiber laser, an ytterbium laser, a CO₂ laser, a YAG laser, or a gasdischarge laser. In some embodiments, the at least one laser emits atleast one wavelength of electromagnetic energy that is strongly absorbedby the ionized medium.

The ignition source can be or can include electrodes, an ultravioletignition source, a capacitive ignition source, an inductive ignitionsource, an RF ignition source, a microwave ignition source, a flashlamp, a pulsed laser, or a pulsed lamp. The ignition source can be acontinuous wave (CW) or pulsed laser impinging on a solid or liquidtarget in the chamber. The ignition source can be external or internalto the chamber.

The light source can include at least one optical element for modifyinga property of electromagnetic radiation emitted by the ionized gas. Theoptical element can be, for example, one or more mirrors or lenses. Insome embodiments, the optical element is configured to deliver theelectromagnetic radiation emitted by the ionized gas to a tool (e.g., awafer inspection tool, a microscope, a metrology tool, a lithographytool, or an endoscopic tool).

The invention, in another aspect, relates to a method for producinglight. The method involves ionizing with an ignition source a gas withina chamber. The method also involves providing laser energy to theionized gas in the chamber to produce a high brightness light.

In some embodiments, the method also involves directing the laser energythrough at least one optical element for modifying a property of thelaser energy provided to the ionized gas. In some embodiments, themethod also involves actively pumping the chamber. The ionizable mediumcan be a moving target. In some embodiments, the method also involvesdirecting the high brightness light through at least one optical elementto modify a property of the light. In some embodiments, the method alsoinvolves delivering the high brightness light emitted by the ionizedmedium to a tool (e.g., a wafer inspection tool, a microscope, ametrology tool, a lithography tool, or an endoscopic tool).

In another aspect, the invention features a light source. The lightssource includes a chamber and an ignition source for ionizing anionizable medium within the chamber. The light source also includes atleast one laser for providing substantially continuous energy to theionized medium within the chamber to produce a high brightness light.

In some embodiments, the at least one laser is a continuous wave laseror a high pulse rate laser. In some embodiments, the at least one laseris a high pulse rate laser that provides pulses of energy to the ionizedmedium so the high brightness light is substantially continuous. In someembodiments, the magnitude of the high brightness light does not vary bymore than about 90% during operation. In some embodiments, the at leastone laser provides energy substantially continuously to minimize coolingof the ionized medium when energy is not provided to the ionized medium.

In some embodiments, the light source can include at least one opticalelement (e.g., a lens or mirror) for modifying a property of the laserenergy provided to the ionized medium. The optical element can be, forexample, an aplanatic lens, an achromatic lens, a single element lens, afresnel lens, a coated mirror, a dielectric coated mirror, a narrow bandmirror, or an ultraviolet transparent infrared reflecting mirror. Insome embodiments, the optical element is one or more fiber opticelements for directing the laser energy to the ionizable medium.

In some embodiments, the chamber includes an ultraviolet transparentregion. In some embodiments, the chamber or a window in the chamberincludes a quartz material, suprasil quartz material, sapphire material,MgF₂ material, diamond material, or CaF₂ material. In some embodiments,the chamber is a sealed chamber. The chamber can be capable of beingactively pumped. In some embodiments, the chamber includes a dielectricmaterial (e.g., quartz). In some embodiments, the chamber is a glassbulb. In some embodiments, the chamber is an ultraviolet transparentdielectric chamber.

The ionizable medium can be a solid, liquid or gas. The ionizable mediumcan include one or more of a noble gas, Xe, Ar, Ne, Kr, He, D₂, H₂, O₂,F₂, a metal halide, a halogen, Hg, Cd, Zn, Sn, Ga, Fe, Li, Na, anexcimer forming gas, air, a vapor, a metal oxide, an aerosol, a flowingmedia, a recycled media, or an evaporating target. In some embodiments,the ionizable medium is a target in the chamber and the ignition sourceis a pulsed laser that provides a pulsed laser beam that strikes thetarget. The target can be a pool or film of metal. In some embodiments,the target is capable of moving.

In some embodiments, the at least one laser is multiple diode laserscoupled into a fiber optic element. The at least one laser can emit atleast one wavelength of electromagnetic energy that is strongly absorbedby the ionized medium.

The ignition source can be or can include electrodes, an ultravioletignition source, a capacitive ignition source, an inductive ignitionsource, an RF ignition source, a microwave ignition source, a flashlamp, a pulsed laser, or a pulsed lamp. The ignition source can beexternal or internal to the chamber.

In some embodiments, the light source includes at least one opticalelement (e.g., a mirror or lens) for modifying a property ofelectromagnetic radiation emitted by the ionized medium. The opticalelement can be configured to deliver the electromagnetic radiationemitted by the ionized medium to a tool (e.g., a wafer inspection tool,a microscope, a metrology tool, a lithography tool, or an endoscopictool).

The invention, in another aspect relates to a method for producinglight. The method involves ionizing with an ignition source an ionizablemedium within a chamber. The method also involves providingsubstantially continuous laser energy to the ionized medium in thechamber to produce a high brightness light.

In some embodiments, the method also involves directing the laser energythrough at least one optical element for modifying a property of thelaser energy provided to the ionizable medium. The method also caninvolve actively pumping the chamber. In some embodiments, the ionizablemedium is a moving target. The ionizable medium can include a solid,liquid or gas. In some embodiments, the method also involves directingthe high brightness light through at least one optical element to modifya property of the light. In some embodiments, the method also involvesdelivering the high brightness light emitted by the ionized medium to atool.

The invention, in another aspect, features a light source having achamber. The light source includes a first ignition means for ionizingan ionizable medium within the chamber. The light source also includes ameans for providing substantially continuous laser energy to the ionizedmedium within the chamber.

The invention, in another aspect, features a light source having achamber that includes a reflective surface. The light source alsoincludes an ignition source for ionizing a gas within the chamber. Thelight source also includes a reflector that at least substantiallyreflects a first set of predefined wavelengths of electromagnetic energydirected toward the reflector and at least substantially allows a secondset of predefined wavelengths of electromagnetic energy to pass throughthe reflector. The light source also includes at least one laser (e.g.,a continuous-wave fiber laser) external to the chamber for providingelectromagnetic energy to the ionized gas within the chamber to producea plasma that generates a high brightness light. A continuous-wave laseremits radiation continuously or substantially continuously rather thanin short bursts, as in a pulsed laser.

In some embodiments, at least one laser directs a first set ofwavelengths of electromagnetic energy through the reflector toward thereflective surface (e.g., inner surface) of the chamber and thereflective surface directs at least a portion of the first set ofwavelengths of electromagnetic energy toward the plasma. In someembodiments, at least a portion of the high brightness light is directedtoward the reflective surface of the chamber, is reflected toward thereflector, and is reflected by the reflector toward a tool. In someembodiments, at least one laser directs a first set of wavelengths ofelectromagnetic energy toward the reflector, the reflector reflects atleast a portion of the first wavelengths of electromagnetic energytowards the reflective surface of the chamber, and the reflectivesurface directs a portion of the first set of wavelengths ofelectromagnetic energy toward the plasma.

In some embodiments, at least a portion of the high brightness light isdirected toward the reflective surface of the chamber, is reflectedtoward the reflector, and passes through the reflector toward an outputof the light source. In some embodiments, the light source comprises amicroscope, ultraviolet microscope, wafer inspection system, reticleinspection system or lithography system spaced relative to the output ofthe light source to receive the high brightness light. In someembodiments, a portion of the high brightness light is directed towardthe reflective surface of the chamber, is reflected toward thereflector, and electromagnetic energy comprising the second set ofpredefined wavelengths of electromagnetic energy passes through thereflector.

The chamber of the light source can include a window. In someembodiments, the chamber is a sealed chamber. In some embodiments, thereflective surface of the chamber comprises a curved shape, parabolicshape, elliptical shape, spherical shape or aspherical shape. In someembodiments, the chamber has a reflective inner surface. In someembodiments, a coating or film is located on the outside of the chamberto produce the reflective surface. In some embodiments, a coating orfilm is located on the inside of the chamber to produce the reflectivesurface. In some embodiments, the reflective surface is a structure oroptical element that is distinct from the inner surface of the chamber.

The light source can include an optical element disposed along a paththe electromagnetic energy from the laser travels. In some embodiments,the optical element is adapted to provide electromagnetic energy fromthe laser to the plasma over a large solid angle. In some embodiments,the reflective surface of the chamber is adapted to provideelectromagnetic energy from the laser to the plasma over a large solidangle. In some embodiments, the reflective surface of the chamber isadapted to collect the high brightness light generated by the plasmaover a large solid angle. In some embodiments, one or more of thereflective surface, reflector and the window include (e.g., are coatedor include) a material to filter predefined wavelengths (e.g., infraredwavelengths of electromagnetic energy) of electromagnetic energy.

The invention, in another aspect, features a light source that includesa chamber that has a reflective surface. The light source also includesan ignition source for ionizing a gas within the chamber. The lightsource also includes at least one laser external to the chamber forproviding electromagnetic energy to the ionized gas within the chamberto produce a plasma that generates a high brightness light. The lightsource also includes a reflector positioned along a path that theelectromagnetic energy travels from the at least one laser to thereflective surface of the chamber.

In some embodiments, the reflector is adapted to at least substantiallyreflect a first set of predefined wavelengths of electromagnetic energydirected toward the reflector and at least substantially allow a secondset of predefined wavelengths of electromagnetic energy to pass throughthe reflector.

The invention, in another aspect, relates to a method for producinglight. The method involves ionizing with an ignition source a gas withina chamber that has a reflective surface. The method also involvesproviding laser energy to the ionized gas in the chamber to produce aplasma that generates a high brightness light.

In some embodiments, the method involves directing the laser energycomprising a first set of wavelengths of electromagnetic energy througha reflector toward the reflective surface of the chamber, the reflectivesurface reflecting at least a portion of the first set of wavelengths ofelectromagnetic energy toward the plasma. In some embodiments, themethod involves directing at least a portion of the high brightnesslight toward the reflective surface of the chamber which is reflectedtoward the reflector and is reflected by the reflector toward a tool.

In some embodiments, the method involves directing the laser energycomprising a first set of wavelengths of electromagnetic energy towardthe reflector, the reflector reflects at least a portion of the firstwavelengths of electromagnetic energy toward the reflective surface ofthe chamber, the reflective surface directs a portion of the first setof wavelengths of electromagnetic energy toward the plasma. In someembodiments, the method involves directing a portion of the highbrightness light toward the reflective surface of the chamber which isreflected toward the reflector and, electromagnetic energy comprisingthe second set of predefined wavelengths of electromagnetic energypasses through the reflector.

The method can involve directing the laser energy through an opticalelement that modifies a property of the laser energy to direct the laserenergy toward the plasma over a large solid angle. In some embodiments,the method involves directing the laser energy through an opticalelement that modifies a property of the laser energy to direct the laserenergy toward the plasma over a solid angle of approximately 0.012steradians. In some embodiments, the method involves directing the laserenergy through an optical element that modifies a property of the laserenergy to direct the laser energy toward the plasma over a solid angleof approximately 0.048 steradians. In some embodiments, the methodinvolves directing the laser energy through an optical element thatmodifies a property of the laser energy to direct the laser energytoward the plasma over a solid angle of greater than about 2π (about6.28) steradians. In some embodiments, the reflective surface of thechamber is adapted to provide the laser energy to the plasma over alarge solid angle. In some embodiments, the reflective surface of thechamber is adapted to collect the high brightness light generated by theplasma over a large solid angle.

The invention, in another aspect, relates to a method for producinglight. The method involves ionizing with an ignition source a gas withina chamber that has a reflective surface. The method also involvesdirecting electromagnetic energy from a laser toward a reflector that atleast substantially reflects a first set of wavelengths ofelectromagnetic energy toward the ionized gas in the chamber to producea plasma that generates a high brightness light.

In some embodiments, the electromagnetic energy from the laser first isreflected by the reflector toward the reflective surface of the chamber.In some embodiments, the electromagnetic energy directed toward thereflective surface of the chamber is reflected toward the plasma. Insome embodiments, a portion of the high brightness light is directedtoward the reflective surface of the chamber, reflected toward thereflector and passes through the reflector.

In some embodiments, the electromagnetic energy from the laser firstpasses through the reflector and travels toward the reflective surfaceof the chamber. In some embodiments, the electromagnetic energy directedtoward the reflective surface of the chamber is reflected toward theplasma. In some embodiments, a portion of the high brightness light isdirected toward the reflective surface of the chamber, reflected towardthe reflector and reflected by the reflector.

The invention, in another aspect, features a light source that includesa chamber having a reflective surface. The light source also includes ameans for ionizing a gas within the chamber. The light source alsoincludes a means for at least substantially reflecting a first set ofpredefined wavelengths of electromagnetic energy directed toward thereflector and at least substantially allowing a second set of predefinedwavelengths of electromagnetic energy to pass through the reflector. Thelight source also includes a means for providing electromagnetic energyto the ionized gas within the chamber to produce a plasma that generatesa high brightness light.

The invention, in another aspect, features a light source that includesa sealed chamber. The light source also includes an ignition source forionizing a gas within the chamber. The light source also includes atleast one laser external to the sealed chamber for providingelectromagnetic energy to the ionized gas within the chamber to producea plasma that generates a high brightness light. The light source alsoincludes a curved reflective surface disposed external to the sealedchamber to receive at leas a portion of the high brightness lightemitted by the sealed chamber and reflect the high brightness lighttoward an output of the light source.

In some embodiments, the light source includes an optical elementdisposed along a path the electromagnetic energy from the laser travels.In some embodiments, the sealed chamber includes a support element thatlocates the sealed chamber relative to the curved reflective surface. Insome embodiments, the sealed chamber is a quartz bulb. In someembodiments, the light source includes a second curved reflectivesurface disposed internal or external to the sealed chamber to receiveat least a portion of the laser electromagnetic energy and focus theelectromagnetic energy on the plasma that generates the high brightnesslight.

The invention, in another aspect, features a light source that includesa sealed chamber and an ignition source for ionizing a gas within thechamber. The light source also includes at least one laser external tothe sealed chamber for providing electromagnetic energy. The lightsource also includes a curved reflective surface to receive and reflectat least a portion of the electromagnetic energy toward the ionized gaswithin the chamber to produce a plasma that generates a high brightnesslight, the curved reflective surface also receives at least a portion ofthe high brightness light emitted by the plasma and reflects the highbrightness light toward an output of the light source.

In some embodiments, the curved reflective surface focuses theelectromagnetic energy on a region in the chamber where the plasma islocated. In some embodiments, the curved reflective surface is locatedwithin the chamber. In some embodiments, the curved reflective surfaceis located external to the chamber. In some embodiments, the highbrightness light is ultraviolet light, includes ultraviolet light or issubstantially ultraviolet light.

The invention, in another aspect, features a light source that includesa chamber. The light source also includes an energy source for providingenergy to a gas within the chamber to produce a plasma that generates alight emitted through the walls of the chamber. The light source alsoincludes a reflector that reflects the light emitted through the wallsof the chamber. The reflector includes a reflective surface with a shapeconfigured to compensate for the refractive index of the walls of thechamber. The shape can include a modified parabolic, elliptical,spherical, or aspherical shape.

In some embodiments, the energy source is at least one laser external tothe chamber. In some embodiments, the energy source is also an ignitionsource within the chamber. The energy source can be a microwave energysource, an AC arc source, a DC arc source, a laser, or an RF energysource. The energy source can be a pulse laser, a continuous-wave fiberlaser, or a diode laser.

In some embodiments, the chamber is a sealed chamber. The chamber caninclude a cylindrical tube. In some embodiments, the cylindrical tube istapered. The chamber can include one or more seals at one or both endsof the cylindrical tube. The chamber can include sapphire, quartz, fusedquartz, Suprasil quartz, fused silica, Suprasil fused silica, MgF₂,diamond, single crystal quartz, or CaF₂. The chamber can include adielectric material. The chamber can include an ultraviolet transparentdielectric material. The chamber can protrude through an opening in thereflector.

In some embodiments, the light source also includes an ignition sourcefor ionizing the gas within the chamber. The ignition source can includeelectrodes, an ultraviolet ignition source, a capacitive ignitionsource, an inductive ignition source, a flash lamp, a pulsed laser, or apulsed lamp. The ignition source can include electrodes located onopposite sides of the plasma.

In some embodiments, the light source also includes a support elementthat locates the chamber relative to the reflector. The support elementcan include a fitting to allow at least one of pressure control orfilling of the chamber.

In some embodiments, the light source includes at least one opticalelement. The optical element can modify a property of the light emittedthrough the walls of the chamber and reflected by the reflector. Theoptical element can be a mirror or a lens. The optical element can beconfigured to deliver the light emitted through the walls of the chamberand reflected by the reflector to a tool (e.g. a wafer inspection tool,a microscope, an ultraviolet microscope, a reticle inspection system, ametrology tool, a lithography tool, or an endoscopic tool).

The invention, in another aspect, features a method for producing light.The method involves emitting a light through the walls of a chamber. Themethod also involves using a reflective surface of a reflector toreflect the light, wherein the reflective surface has a shape configuredto compensate for the refractive index of the walls of the chamber.

In some embodiments, the method also involves flowing gas into thechamber. In some embodiments, the method also involves igniting the gasin the chamber to produce an ionized gas. In some embodiments, themethod also involves directing energy to the ionized gas to produce aplasma that generates a light (e.g. a high brightness light). In someembodiments, the method also involves directing laser energy into thechamber from at least one laser external to the chamber. In someembodiments, the method also involves directing the laser energy throughan optical element that modifies a property of the laser energy. In someembodiments, the method also involves directing the reflected lightthrough an optical element to modify a property of the reflected light.In some embodiments, the method also involves directing the reflectedlight to a tool. In some embodiments, the method also involvescontrolling the pressure of the chamber.

In some embodiments, the method also involves expressing the shape as amathematical equation. In some embodiments, the method also involvesselecting parameters of the equation to reduce error due to therefractive index of the walls of the chamber below a specified value. Insome embodiments, the method also involves configuring the shape tocompensate for the refractive index of the walls of the chamber. In someembodiments, the method also involves producing a collimated or focusedbeam of reflected light with the reflective surface. In someembodiments, the method also involves modifying a parabolic, elliptical,spherical, or aspherical shape to compensate for the refractive index ofthe walls of the chamber to produce a focused, reflected high brightnesslight.

The invention, in another aspect, features a light source including achamber. The light source also includes a laser source for providingelectromagnetic energy to a gas within the chamber to produce a plasmathat generates a light emitted through the walls of the chamber. Thelight source also includes a reflector that reflects the electromagneticenergy through the walls of the chamber and the light emitted throughthe walls of the chamber, the reflector includes a reflective surfacewith a shape configured to compensate for the refractive index of thewalls of the chamber.

The invention, in another aspect, features a light source having achamber. The light source also includes means for providing energy to agas within the chamber to produce a plasma that generates a lightemitted through the walls of the chamber. The light source also includesmeans for reflecting the light emitted through the walls of the chamber,the reflecting means including a reflective surface with a shapeconfigured to compensate for the refractive index of the walls of thechamber.

The invention, in another aspect, features a light source having achamber. The light source also includes an ignition source for ionizinga medium (e.g., a gas) within the chamber. The light source alsoincludes a laser for providing energy to the ionized medium within thechamber to produce a light. The light source also includes a blockersuspended along a path the energy travels to block at least a portion ofthe energy.

In some embodiments, the blocker deflects energy provided to the ionizedmedium that is not absorbed by the ionized medium away from an output ofthe light source. In some embodiments, the blocker is a mirror.

In some embodiments, the blocker absorbs the energy provided to theionized medium that is not absorbed by the ionized medium. The blockercan include graphite.

In some embodiments, the blocker reflects energy provided to the ionizedmedium that is not absorbed by the ionized medium. In some embodiments,the reflected energy is reflected toward the ionized medium in thechamber. In some embodiments, the blocker is a coating on a portion ofthe chamber.

In some embodiments, the light source includes a coolant channeldisposed in the blocker. In some embodiments, the light source includesa coolant supply (e.g., for supplying coolant, for example, water)coupled to the coolant channel. In some embodiments, light sourceincludes a gas source that blows a gas (e.g., nitrogen or air) on theblocker to cool the blocker.

In some embodiments, the light source includes an arm connecting theblocker to a housing of the light source.

In some embodiments, the energy provided by the laser enters the chamberon a first side of the chamber and the blocker is suspended on a secondside of the chamber opposite the first side.

The invention, in another aspect, relates to a method for producinglight. The method involves ionizing with an ignition source a mediumwithin a chamber. The method also involves providing laser energy to theionized medium in the chamber to produce a light. The method alsoinvolves blocking energy provided to the ionized medium that is notabsorbed by the ionized medium with a blocker suspended along a path theenergy travels.

In some embodiments, blocking the energy involves deflecting the energyaway from an output of the light source. In some embodiments, theblocker includes a mirror. In some embodiments, blocking the energyincludes absorbing the energy. In some embodiments, blocking the energyincludes reflecting the energy. In some embodiments, reflecting theenergy includes reflecting the energy towards the ionized medium in thechamber.

In some embodiments, the method also involves cooling the blocker. Insome embodiments, cooling the blocker includes flowing a coolant througha channel in or coupled to the blocker. In some embodiments, the methodinvolves blowing a gas on the blocker to cooler the blocker.

The invention, in another aspect, relates to a method for producinglight. The method involves ionizing with an ignition source a gas withina chamber. The method also involves providing laser energy to theionized gas in the chamber at a pressure of greater than 10 atmospheresto produce a high brightness light.

In some embodiments, the gas within the chamber is at a pressure ofgreater than 30 atmospheres. In some embodiments, the gas within thechamber is at a pressure of greater than 50 atmospheres. In someembodiments, the high brightness light is emitted from a plasma having avolume of about 0.01 mm³.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, feature and advantages of theinvention, as well as the invention itself, will be more fullyunderstood from the following illustrative description, when readtogether with the accompanying drawings which are not necessarily toscale.

FIG. 1 is a schematic block diagram of a light source, according to anillustrative embodiment of the invention.

FIG. 2 is a schematic block diagram of a portion of a light source,according to an illustrative embodiment of the invention.

FIG. 3 is a graphical representation of UV brightness as a function ofthe laser power provided to a plasma, using a light source according tothe invention.

FIG. 4 is a graphical representation of the transmission of laser energythrough a plasma generated from mercury, using a light source accordingto the invention.

FIG. 5 is a schematic block diagram of a light source, according to anillustrative embodiment of the invention.

FIG. 6 is a schematic block diagram of a light source, according to anillustrative embodiment of the invention.

FIG. 7 is a schematic block diagram of a light source, according to anillustrative embodiment of the invention.

FIG. 8A is a schematic block diagram of a light source in whichelectromagnetic energy from a laser is provided to a plasma over a firstsolid angle, according to an illustrative embodiment of the invention.

FIG. 8B is a schematic block diagram of the light source of FIG. 8A inwhich the electromagnetic energy from the laser is provided to theplasma over a larger solid angle, according to an illustrativeembodiment of the invention.

FIG. 9 is a schematic diagram of a light source in which the reflectivesurface does not compensate for the refractive index of the chambercontaining a plasma.

FIG. 10A is a schematic diagram of a light source with a chamber and areflector that reflects light produced in the chamber according, to anillustrative embodiment of the invention.

FIG. 10B is a schematic diagram of a light source with a chamber and areflector that reflects light produced in the chamber according, to anillustrative embodiment of the invention.

FIG. 11 is a schematic diagram of a light source including a chamber anda reflector that reflects light produced in the chamber, according to anillustrative embodiment of the invention.

FIG. 12 is a cross-sectional view of a light source, according to anillustrative embodiment of the invention.

FIG. 13 is a graphical representation of the radii by which light raysreflected off of the reflective surface miss the remote focus point fordifferent mathematical fit orders for a mathematical equation thatexpresses the shape of the reflective surface.

FIG. 14 is a schematic block diagram of a light source, according to anillustrative embodiment of the invention.

FIG. 15A is a cross-sectional view of a light source, according to anillustrative embodiment of the invention.

FIG. 15B is an end face view of the light source of FIG. 15A.

FIG. 16 is a graphical representation of brightness as a function of thepressure in a chamber of a light source, using a light source accordingto the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 is a schematic block diagram of a light source 100 for generatinglight, that embodies the invention. The light source 100 includes achamber 128 that contains an ionizable medium (not shown). The lightsource 100 provides energy to a region 130 of the chamber 128 having theionizable medium which creates a plasma 132. The plasma 132 generatesand emits a high brightness light 136 that originates from the plasma132. The light source 100 also includes at least one laser source 104that generates a laser beam that is provided to the plasma 132 locatedin the chamber 128 to initiate and/or sustain the high brightness light136.

In some embodiments, it is desirable for at least one wavelength ofelectromagnetic energy generated by the laser source 104 to be stronglyabsorbed by the ionizable medium in order to maximize the efficiency ofthe transfer of energy from the laser source 104 to the ionizablemedium.

In some embodiments, it is desirable for the plasma 132 to be small insize in order to achieve a high brightness light source. Brightness isthe power radiated by a source of light per unit surface area into aunit solid angle. The brightness of the light produced by a light sourcedetermines the ability of a system (e.g., a metrology tool) or anoperator to see or measure things (e.g., features on the surface of awafer) with adequate resolution. It is also desirable for the lasersource 104 to drive and/or sustain the plasma with a high power laserbeam.

Generating a plasma 132 that is small in size and providing the plasma132 with a high power laser beam leads simultaneously to a highbrightness light 136. The light source 100 produces a high brightnesslight 136 because most of the power introduced by the laser source 104is then radiated from a small volume, high temperature plasma 132. Theplasma 132 temperature will rise due to heating by the laser beam untilbalanced by radiation and other processes. The high temperatures thatare achieved in the laser sustained plasma 132 yield increased radiationat shorter wavelengths of electromagnetic energy, for example,ultraviolet energy. In one experiment, temperatures between about 10,000K and about 20,000 K have been observed. The radiation of the plasma132, in a general sense, is distributed over the electromagneticspectrum according to Planck's radiation law. The wavelength of maximumradiation is inversely proportional to the temperature of a black bodyaccording to Wien's displacement law. While the laser sustained plasmais not a black body, it behaves similarly and as such, the highestbrightness in the ultraviolet range at around 300 nm wavelength isexpected for laser sustained plasmas having a temperature of betweenabout 10,000 K and about 15,000 K. Most conventional arc lamps are,however, unable to operate at these temperatures.

It is therefore desirable in some embodiments of the invention tomaintain the temperature of the plasma 132 during operation of the lightsource 100 to ensure that a sufficiently bright light 136 is generatedand that the light emitted is substantially continuous during operation.

In this embodiment, the laser source 104 is a diode laser that outputs alaser beam via a fiberoptic element 108. The fiber optic element 108provides the laser beam to a collimator 112 that aids in conditioningthe output of the diode laser by aiding in making laser beam rays 116substantially parallel to each other. The collimator 112 then directsthe laser beam 116 to a beam expander 118. The beam expander 118 expandsthe size of the laser beam 116 to produce laser beam 122. The beamexpander 118 also directs the laser beam 122 to an optical lens 120. Theoptical lens 120 is configured to focus the laser beam 122 to produce asmaller diameter laser beam 124 that is directed to the region 130 ofthe chamber 128 where the plasma 132 exists (or where it is desirablefor the plasma 132 to be generated and sustained).

In this embodiment, the light source 100 also includes an ignitionsource 140 depicted as two electrodes (e.g., an anode and cathodelocated in the chamber 128). The ignition source 140 generates anelectrical discharge in the chamber 128 (e.g., the region 130 of thechamber 128) to ignite the ionizable medium. The laser then provideslaser energy to the ionized medium to sustain or create the plasma 132which generates the high brightness light 136. The light 136 generatedby the light source 100 is then directed out of the chamber to, forexample, a wafer inspection system (not shown).

Alternative laser sources are contemplated according to illustrativeembodiments of the invention. In some embodiments, neither thecollimator 112, the beam expander 118, or the lens 120 may be required.In some embodiments, additional or alternative optical elements can beused. The laser source can be, for example, an infrared (IR) lasersource, a diode laser source, a fiber laser source, an ytterbium lasersource, a CO₂ laser source, a YAG laser source, or a gas discharge lasersource. In some embodiments, the laser source 104 is a pulse lasersource (e.g., a high pulse rate laser source) or a continuous wave lasersource. Fiber lasers use laser diodes to pump a special doped fiberwhich then lases to produce the output (i.e., a laser beam). In someembodiments, multiple lasers (e.g., diode lasers) are coupled to one ormore fiber optic elements (e.g., the fiber optic element 108). Diodelasers take light from one, or usually many, diodes and directs thelight down a fiber to the output. In some embodiments, fiber lasersources and direct semiconductor laser sources are desirable for use asthe laser source 104 because they are relatively low in cost, have asmall form factor or package size, and are relatively high inefficiency.

Efficient, cost effective, high power lasers (e.g., fiber lasers anddirect diode lasers) are recently available in the NIR (near infrared)wavelength range from about 700 nm to about 2000 nm. Energy in thiswavelength range is more easily transmitted through certain materials(e.g., glass, quartz and sapphire) that are more commonly used tomanufacture bulbs, windows and chambers. It is therefore more practicalnow to produce light sources that operate using lasers in the 700 nm to2000 nm range than has previously been possible.

In some embodiments, the laser source 104 is a high pulse rate lasersource that provides substantially continuous laser energy to the lightsource 100 sufficient to produce the high brightness light 136. In someembodiments, the emitted high brightness light 136 is substantiallycontinuous where, for example, magnitude (e.g. brightness or power) ofthe high brightness light does not vary by more than about 90% duringoperation. In some embodiments, the ratio of the peak power of the laserenergy delivered to the plasma to the average power of the laser energydelivered to the plasma is approximately 2-3. In some embodiments, thesubstantially continuous energy provided to the plasma 132 is sufficientto minimize cooling of the ionized medium to maintain a desirablebrightness of the emitted light 136.

In this embodiment, the light source 100 includes a plurality of opticalelements (e.g., a beam expander 118, a lens 120, and fiber optic element108) to modify properties (e.g., diameter and orientation) of the laserbeam delivered to the chamber 132. Various properties of the laser beamcan be modified with one or more optical elements (e.g., mirrors orlenses). For example, one or more optical elements can be used to modifythe portions of, or the entire laser beam diameter, direction,divergence, convergence, and orientation. In some embodiments, opticalelements modify the wavelength of the laser beam and/or filter outcertain wavelengths of electromagnetic energy in the laser beam.

Lenses that can be used in various embodiments of the invention include,aplanatic lenses, achromatic lenses, single element lenses, and fresnellenses. Mirrors that can be used in various embodiments of the inventioninclude, coated mirrors, dielectric coated mirrors, narrow band mirrors,and ultraviolet transparent infrared reflecting mirrors. By way ofexample, ultraviolet transparent infrared reflecting mirrors are used insome embodiments of the invention where it is desirable to filter outinfrared energy from a laser beam while permitting ultraviolet energy topass through the mirror to be delivered to a tool (e.g., a waferinspection tool, a microscope, a lithography tool or an endoscopictool).

In this embodiment, the chamber 128 is a sealed chamber initiallycontaining the ionizable medium (e.g., a solid, liquid or gas). In someembodiments, the chamber 128 is instead capable of being actively pumpedwhere one or more gases are introduced into the chamber 128 through agas inlet (not shown), and gas is capable of exiting the chamber 128through a gas outlet (not shown). The chamber can be fabricated from orinclude one or more of, for example, a dielectric material, a quartzmaterial, Suprasil quartz, sapphire, MgF₂, diamond or CaF₂. The type ofmaterial may be selected based on, for example, the type of ionizablemedium used and/or the wavelengths of light 136 that are desired to begenerated and output from the chamber 128. In some embodiments, a regionof the chamber 128 is transparent to, for example, ultraviolet energy.Chambers 128 fabricated using quartz will generally allow wavelengths ofelectromagnetic energy of as long as about 2 microns to pass throughwalls of the chamber. Sapphire chamber walls generally allowelectromagnetic energy of as long as about 4 microns to pass through thewalls.

In some embodiments, it is desirable for the chamber 128 to be a sealedchamber capable of sustaining high pressures and temperatures. Forexample, in one embodiment, the ionizable medium is mercury vapor. Tocontain the mercury vapor during operation, the chamber 128 is a sealedquartz bulb capable of sustaining pressures between about 10 to about200 atmospheres and operating at about 900 degrees centigrade. Thequartz bulb also allows for transmission of the ultraviolet light 136generated by the plasma 132 of the light source 100 through the chamber128 walls.

Various ionizable media can be used in alternative embodiments of theinvention. For example, the ionizable medium can be one or more of anoble gas, Xe, Ar, Ne, Kr, He, D₂, H₂, O₂, F₂, a metal halide, ahalogen, Hg, Cd, Zn, Sn, Ga, Fe, Li, Na, an excimer forming gas, air, avapor, a metal oxide, an aerosol, a flowing media, or a recycled media.In some embodiments, a solid or liquid target (not shown) in the chamber128 is used to generate an ionizable gas in the chamber 128. The lasersource 104 (or an alternative laser source) can be used to provideenergy to the target to generate the ionizable gas. The target can be,for example, a pool or film of metal. In some embodiments, the target isa solid or liquid that moves in the chamber (e.g., in the form ofdroplets of a liquid that travel through the region 130 of the chamber128). In some embodiments, a first ionizable gas is first introducedinto the chamber 128 to ignite the plasma 132 and then a separate secondionizable gas is introduced to sustain the plasma 132. In thisembodiment, the first ionizable gas is a gas that is more easily ignitedusing the ignition source 140 and the second ionizable gas is a gas thatproduces a particular wavelength of electromagnetic energy.

In this embodiment, the ignition source 140 is a pair of electrodeslocated in the chamber 128. In some embodiments, the electrodes arelocated on the same side of the chamber 128. A single electrode can beused with, for example, an RF ignition source or a microwave ignitionsource. In some embodiments, the electrodes available in a conventionalarc lamp bulb are the ignition source (e.g., a model USH-200DP quartzbulb manufactured by Ushio (with offices in Cypress, Calif.)). In someembodiments, the electrodes are smaller and/or spaced further apart thanthe electrodes used in a conventional arc lamp bulb because theelectrodes are not required for sustaining the high brightness plasma inthe chamber 128.

Various types and configurations of ignition sources are alsocontemplated, however, that are within the scope of the presentinvention. In some embodiments, the ignition source 140 is external tothe chamber 128 or partially internal and partially external to thechamber 128. Alternative types of ignition sources 140 that can be usedin the light source 100 include ultraviolet ignition sources, capacitivedischarge ignition sources, inductive ignition sources, RF ignitionsources, a microwave ignition sources, flash lamps, pulsed lasers, andpulsed lamps. In one embodiment, no ignition source 140 is required andinstead the laser source 104 is used to ignite the ionizable medium andto generate the plasma 132 and to sustain the plasma and the highbrightness light 136 emitted by the plasma 132.

In some embodiments, it is desirable to maintain the temperature of thechamber 128 and the contents of the chamber 128 during operation of thelight source 100 to ensure that the pressure of gas or vapor within thechamber 128 is maintained at a desired level. In some embodiments, theignition source 140 can be operated during operation of the light source100, where the ignition source 140 provides energy to the plasma 132 inaddition to the energy provided by the laser source 104. In this manner,the ignition source 140 is used to maintain (or maintain at an adequatelevel) the temperature of the chamber 128 and the contents of thechamber 128.

In some embodiments, the light source 100 includes at least one opticalelement (e.g., at least one mirror or lens) for modifying a property ofthe electromagnetic energy (e.g., the high brightness light 136) emittedby the plasma 132 (e.g., an ionized gas), similarly as describedelsewhere herein.

FIG. 2 is a schematic block diagram of a portion of a light source 200incorporating principles of the present invention. The light source 200includes a chamber 128 containing an ionizable gas and has a window 204that maintains a pressure within the chamber 128 while also allowingelectromagnetic energy to enter the chamber 128 and exit the chamber128. In this embodiment, the chamber 128 has an ignition source (notshown) that ignites the ionizable gas (e.g., mercury or xenon) toproduce a plasma 132.

A laser source 104 (not shown) provides a laser beam 216 that isdirected through a lens 208 to produce laser beam 220. The lens 208focuses the laser beam 220 on to a surface 224 of a thin film reflector212 that reflects the laser beam 220 to produce laser beam 124. Thereflector 212 directs the laser beam 124 on region 130 where the plasma132 is located. The laser beam 124 provides energy to the plasma 132 tosustain and/or generate a high brightness light 136 that is emitted fromthe plasma 132 in the region 130 of the chamber 128.

In this embodiment, the chamber 128 has a paraboloid shape and an innersurface 228 that is reflective. The paraboloid shape and the reflectivesurface cooperate to reflect a substantial amount of the high brightnesslight 136 toward and out of the window 204. In this embodiment, thereflector 212 is transparent to the emitted light 136 (e.g., at leastone or more wavelengths of ultraviolet light). In this manner, theemitted light 136 is transmitted out of the chamber 128 and directed to,for example, a metrology tool (not shown). In one embodiment, theemitted light 136 is first directed towards or through additionaloptical elements before it is directed to a tool.

By way of illustration, an experiment was conducted to generateultraviolet light using a light source, according to an illustrativeembodiment of the invention. A model L6724 quartz bulb manufactured byHamamatsu (with offices in Bridgewater, N.J.) was used as the chamber ofthe light source (e.g., the chamber 128 of the light source 100 ofFIG. 1) for experiments using xenon as the ionizable medium in thechamber. A model USH-200DP quartz bulb manufactured by Ushio (withoffices in Cypress, Calif.) was used as the chamber of the light sourcefor experiments using mercury as the ionizable medium in the chamber.FIG. 3 illustrates a plot 300 of the UV brightness of a high brightnesslight produced by a plasma located in the chamber as a function of thelaser power (in watts) provided to the plasma. The laser source used inthe experiment was a 1.09 micron, 100 watt CW laser. The Y-Axis 312 ofthe plot 300 is the UV brightness (between about 200 and about 400 nm)in watts/mm² steradian (sr). The X-Axis 316 of the plot 300 is the laserbeam power in watts provided to the plasma. Curve 304 is the UVbrightness of the high brightness light produced by a plasma that wasgenerated using xenon as the ionizable medium in the chamber. The plasmain the experiment using xenon was between about 1 mm and about 2 mm inlength and about 0.1 mm in diameter. The length of the plasma wascontrolled by adjusting the angle of convergence of the laser beam. Alarger angle (i.e., larger numerical aperture) leads to a shorter plasmabecause the converging beam reaches an intensity capable of sustainingthe plasma when it is closer to the focal point. Curve 308 is the UVbrightness of the high brightness light produced by a plasma that wasgenerated using mercury as the ionizable medium in the chamber. Theplasma in the experiment using mercury was about 1 mm in length andabout 0.1 mm in diameter.

By way of illustration, another experiment was conducted to generateultraviolet using a light source according to an illustrative embodimentof the invention. A model USH-200DP quartz bulb manufactured by Ushio(with offices in Cypress, Calif.) was used as the chamber of the lightsource for experiments using mercury as the ionizable medium in thechamber (e.g., the chamber 128 of the light source 100 of FIG. 1). Thelaser source used in the experiment was a 1.09 micron, 100 wattytterbium doped fiber laser from SPI Lasers PLC (with offices in LosGatos, Calif.). FIG. 4 illustrates a plot 400 of the transmission oflaser energy through a plasma located in the chamber generated frommercury versus the amount of power provided to the plasma in watts. TheY-Axis 412 of the plot 400 is the transmission coefficient innon-dimensional units. The X-Axis 416 of the plot 400 is the laser beampower in watts provided to the plasma. The curve in the plot 400illustrates absorption lengths of 1 mm were achieved using the lasersource. The transmission value of 0.34 observed at 100 watts correspondsto a 1/e absorption length of about 1 mm.

FIG. 5 is a schematic block diagram of a portion of a light source 500incorporating principles of the present invention. The light source 500includes a chamber 528 that has a reflective surface 540. The reflectivesurface 540 can have, for example, a parabolic shape, elliptical shape,curved shape, spherical shape or aspherical shape. In this embodiment,the light source 500 has an ignition source (not shown) that ignites anionizable gas (e.g., mercury or xenon) in a region 530 within thechamber 528 to produce a plasma 532.

In some embodiments, the reflective surface 540 can be a reflectiveinner or outer surface. In some embodiments, a coating or film islocated on the inside or outside of the chamber to produce thereflective surface 540.

A laser source (not shown) provides a laser beam 516 that is directedtoward a surface 524 of a reflector 512. The reflector 512 reflects thelaser beam 520 toward the reflective surface 540 of the chamber 528. Thereflective surface 540 reflects the laser beam 520 and directs the laserbeam toward the plasma 532. The laser beam 516 provides energy to theplasma 532 to sustain and/or generate a high brightness light 536 thatis emitted from the plasma 532 in the region 530 of the chamber 528. Thehigh brightness light 536 emitted by the plasma 532 is directed towardthe reflective surface 540 of the chamber 528. At least a portion of thehigh brightness light 536 is reflected by the reflective surface 540 ofthe chamber 528 and directed toward the reflector 512. The reflector 512is substantially transparent to the high brightness light 536 (e.g., atleast one or more wavelengths of ultraviolet light). In this manner, thehigh brightness light 536 passes through the reflector 512 and isdirected to, for example, a metrology tool (not shown). In someembodiments, the high brightness light 536 is first directed towards orthrough a window or additional optical elements before it is directed toa tool.

In some embodiments, the light source 500 includes a separate, sealedchamber (e.g., the sealed chamber 728 of FIG. 7) located in the concaveregion of the chamber 528. The sealed chamber contains the ionizable gasthat is used to create the plasma 532. In alternative embodiments, thesealed chamber contains the chamber 528. In some embodiments, the sealedchamber also contains the reflector 512.

FIG. 6 is a schematic block diagram of a portion of a light source 600incorporating principles of the present invention. The light source 600includes a chamber 628 that has a reflective surface 640. The reflectivesurface 640 can have, for example, a parabolic shape, elliptical shape,curved shape, spherical shape or aspherical shape. In this embodiment,the light source 600 has an ignition source (not shown) that ignites anionizable gas (e.g., mercury or xenon) in a region 630 within thechamber 628 to produce a plasma 632.

A laser source (not shown) provides a laser beam 616 that is directedtoward a reflector 612. The reflector 612 is substantially transparentto the laser beam 616. The laser beam 616 passes through the reflector612 and is directed toward the reflective surface 640 of the chamber628. The reflective surface 640 reflects the laser beam 616 and directsit toward the plasma 632 in the region 630 of the chamber 628. The laserbeam 616 provides energy to the plasma 632 to sustain and/or generate ahigh brightness light 636 that is emitted from the plasma 632 in theregion 630 of the chamber 628. The high brightness light 636 emitted bythe plasma 632 is directed toward the reflective surface 640 of thechamber 628. At least a portion of the high brightness light 636 isreflected by the reflective surface 640 of the chamber 628 and directedtoward a surface 624 of the reflector 612. The reflector 612 reflectsthe high brightness light 636 (e.g., at least one or more wavelengths ofultraviolet light). In this manner, the high brightness light 636 (e.g.,visible and/or ultraviolet light) is directed to, for example, ametrology tool (not shown). In some embodiments, the high brightnesslight 636 is first directed towards or through a window or additionaloptical elements before it is directed to a tool. In some embodiments,the high brightness light 636 includes ultraviolet light. Ultravioletlight is electromagnetic energy with a wavelength shorter than that ofvisible light, for instance between about 50 nm and 400 nm.

In some embodiments, the light source 600 includes a separate, sealedchamber (e.g., the sealed chamber 728 of FIG. 7) located in the concaveregion of the chamber 628. The sealed chamber contains the ionizable gasthat is used to create the plasma 632. In alternative embodiments, thesealed chamber contains the chamber 628. In some embodiments, the sealedchamber also contains the reflector 612.

FIG. 7 is a schematic block diagram of a light source 700 for generatinglight, that embodies the invention. The light source 700 includes asealed chamber 728 (e.g., a sealed quartz bulb) that contains anionizable medium (not shown). The light source 700 provides energy to aregion 730 of the chamber 728 having the ionizable medium which createsa plasma 732. The plasma 732 generates and emits a high brightness light736 that originates from the plasma 732. The light source 700 alsoincludes at least one laser source 704 that generates a laser beam thatis provided to the plasma 732 located in the chamber 728 to initiateand/or sustain the high brightness light 736.

In this embodiment, the laser source 704 is a diode laser that outputs alaser beam via a fiberoptic element 708. The fiber optic element 708provides the laser beam to a collimator 712 that aids in conditioningthe output of the diode laser by aiding in making laser beam rays 716substantially parallel to each other. The collimator 712 then directsthe laser beam 716 to a beam expander 718. The beam expander 718 expandsthe size of the laser beam 716 to produce laser beam 722. The beamexpander 718 also directs the laser beam 722 to an optical lens 720. Theoptical lens 720 is configured to focus the laser beam 722 to produce asmaller diameter laser beam 724. The laser beam 724 passes through anaperture or window 772 located in the base 724 of a curved reflectivesurface 740 and is directed toward the chamber 728. The chamber 728 issubstantially transparent to the laser beam 724. The laser beam 724passes through the chamber 728 and toward the region 730 of the chamber728 where the plasma 732 exists (or where it is desirable for the plasma732 to be generated by the laser 724 and sustained).

In this embodiment, the ionizable medium is ignited by the laser beam724. In alternative embodiments, the light source 700 includes anignition source (e.g., a pair of electrodes or a source of ultravioletenergy) that, for example, generates an electrical discharge in thechamber 728 (e.g., the region 730 of the chamber 728) to ignite theionizable medium. The laser source 704 then provides laser energy to theionized medium to sustain the plasma 732 which generates the highbrightness light 736. The chamber 728 is substantially transparent tothe high brightness light 736 (or to predefined wavelengths ofelectromagnetic radiation in the high brightness light 736). The light736 (e.g., visible and/or ultraviolet light) generated by the lightsource 700 is then directed out of the chamber 728 toward an innersurface 744 of the reflective surface 740.

In this embodiment, the light source 700 includes a plurality of opticalelements (e.g., a beam expander 718, a lens 720, and fiber optic element708) to modify properties (e.g., diameter and orientation) of the laserbeam delivered to the chamber 732. Various properties of the laser beamcan be modified with one or more optical elements (e.g., mirrors orlenses). For example, one or more optical elements can be used to modifythe portions of, or the entire laser beam diameter, direction,divergence, convergence, and orientation. In some embodiments, opticalelements modify the wavelength of the laser beam and/or filter outcertain wavelengths of electromagnetic energy in the laser beam.

Lenses that can be used in various embodiments of the invention include,aplanatic lenses, achromatic lenses, single element lenses, and fresnellenses. Mirrors that can be used in various embodiments of the inventioninclude, coated mirrors, dielectric coated mirrors, narrow band mirrors,and ultraviolet transparent infrared reflecting mirrors. By way ofexample, ultraviolet transparent infrared reflecting mirrors are used insome embodiments of the invention where it is desirable to filter outinfrared energy from a laser beam while permitting ultraviolet energy topass through the mirror to be delivered to a tool (e.g., a waferinspection tool, a microscope, a lithography tool or an endoscopictool).

FIGS. 8A and 8B are schematic block diagrams of a light source 800 forgenerating light, that embodies the invention. The light source 800includes a chamber 828 that contains an ionizable medium (not shown).The light source 800 provides energy to a region 830 of the chamber 828having the ionizable medium which creates a plasma. The plasma generatesand emits a high brightness light that originates from the plasma. Thelight source 800 also includes at least one laser source 804 thatgenerates a laser beam that is provided to the plasma located in thechamber 828 to initiate and/or sustain the high brightness light.

In some embodiments, it is desirable for the plasma to be small in sizein order to achieve a high brightness light source. Brightness is thepower radiated by a source of light per unit surface area into a unitsolid angle. The brightness of the light produced by a light sourcedetermines the ability of a system (e.g., a metrology tool) or anoperator to see or measure things (e.g., features on the surface of awafer) with adequate resolution. It is also desirable for the lasersource 804 to drive and/or sustain the plasma with a high power laserbeam.

Generating a plasma that is small in size and providing the plasma witha high power laser beam leads simultaneously to a high brightness light.The light source 800 produces a high brightness light because most ofthe power introduced by the laser source 804 is then radiated from asmall volume, high temperature plasma. The plasma temperature will risedue to heating by the laser beam until balanced by radiation and otherprocesses. The high temperatures that are achieved in the lasersustained plasma yield increased radiation at shorter wavelengths ofelectromagnetic energy, for example, ultraviolet energy. In oneexperiment, temperatures between about 10,000 K and about 20,000 K havebeen observed. The radiation of the plasma, in a general sense, isdistributed over the electromagnetic spectrum according to Planck'sradiation law. The wavelength of maximum radiation is inverselyproportional to the temperature of a black body according to Wien'sdisplacement law. While the laser sustained plasma is not a black body,it behaves similarly and as such, the highest brightness in theultraviolet range at around 300 nm wavelength is expected for lasersustained plasmas having a temperature of between about 10,000 K andabout 15,000 K. Conventional arc lamps are, however, unable to operateat these temperatures.

It is desirable in some embodiments of the invention to deliver thelaser energy to the plasma in the chamber 828 over a large solid anglein order to achieve a plasma that is small in size. Various methods andoptical elements can be used to deliver the laser energy over a largesolid angle. In this embodiment of the invention, parameters of a beamexpander and optical lens are varied to modify the size of the solidangle over which the laser energy is delivered to the plasma in thechamber 828.

Referring to FIG. 8A, the laser source 804 is a diode laser that outputsa laser beam via a fiberoptic element 808. The fiber optic element 808provides the laser beam to a collimator 812 that aids in conditioningthe output of the diode laser by aiding in making laser beam rays 816substantially parallel to each other. The collimator 812 directs thelaser beam 816 to an optical lens 820. The optical lens 820 isconfigured to focus the laser beam 816 to produce a smaller diameterlaser beam 824 having a solid angle 878. The laser beam 824 is directedto the region 830 of the chamber 828 where the plasma 832 exists.

In this embodiment, the light source 800 also includes an ignitionsource 840 depicted as two electrodes (e.g., an anode and cathodelocated in the chamber 828). The ignition source 840 generates anelectrical discharge in the chamber 828 (e.g., the region 830 of thechamber 828) to ignite the ionizable medium. The laser then provideslaser energy to the ionized medium to sustain or create the plasma 832which generates the high brightness light 836. The light 836 generatedby the light source 800 is then directed out of the chamber to, forexample, a wafer inspection system (not shown).

FIG. 8B illustrates an embodiment of the invention in which the laserenergy is delivered to the plasma in the chamber 828 over a solid angle874. This embodiment of the invention includes a beam expander 854. Thebeam expander 854 expands the size of the laser beam 816 to producelaser beam 858. The beam expander 854 directs the laser beam 858 to anoptical lens 862. The combination of the beam expander 854 and theoptical lens 862 produces a laser beam 866 that has a solid angle 874that is larger than the solid angle 878 of the laser beam 824 of FIG.8A. The larger solid angle 874 of FIG. 8B creates a smaller size plasma884 than the size of the plasma in FIG. 8A. In this embodiment, the sizeof the plasma 884 in FIG. 8B along the X-axis and Y-axis is smaller thanthe size of the plasma 832 in FIG. 8A. In this manner, the light source800 generates a brighter light 870 in FIG. 8B as compared with the light836 in FIG. 8A.

An experiment was conducted in which a beam expander and optical lenswere selected to allow operation of the light source as shown in FIGS.8A and 8B. A Hamamatsu L2273 xenon bulb (with offices in Bridgewater,N.J.) was used as the sealed chamber 828. The plasma was formed in theHamamatsu L2273 xenon bulb using an SPI continuous-wave (CW) 100 W, 1090nm fiber laser (sold by SPI Lasers PLC, with offices in Los Gatos,Calif.)). A continuous-wave laser emits radiation continuously orsubstantially continuously rather than in short bursts, as in a pulsedlaser. The fiber laser 804 contains laser diodes which are used to pumpa special doped fiber (within the fiber laser 804, but not shown). Thespecial doped fiber then lases to produce the output of the fiber laser804. The output of the fiber laser 804 then travels through thefiberoptic element 808 to the collimeter 812. The collimeter 812 thenoutputs the laser beam 816. The initial laser beam diameter (along theY-Axis), corresponding to beam 816 in FIG. 8A, was 5 mm. The laser beam816 was a Gaussian beam with a 5 mm diameter measured to the

$\frac{1}{{\mathbb{e}}^{2}}$intensity level. The lens used in the experiment, corresponding to lens820, was 30 mm in diameter and had a focal length of 40 mm. Thisproduced a solid angle of illumination of the plasma 832 ofapproximately 0.012 steradians. The length (along the X-Axis) of theplasma 832 produced in this arrangement was measured to be approximately2 mm. The diameter of the plasma 832 (along the Y-Axis), wasapproximately 0.05 mm. The plasma 832 generated a high brightnessultraviolet light 836.

Referring to FIG. 8B, a 2× beam expander was used as the beam expander854. The beam expander 854 expanded beam 816 from 5 mm in diameter(along the Y-Axis) to 10 mm in diameter, corresponding to beam 858. Lens862 in FIG. 8B was the same as lens 820 in FIG. 8A. The combination ofthe beam expander 854 and the optical lens 862 produced a laser beam 866having a solid angle 874 of illumination of approximately 0.048steradians. In this experiment, the length of the plasma (along theX-Axis) was measured to be approximately 1 mm and the diameter measuredalong the Y-Axis remained 0.05 mm. This reduction of plasma length by afactor of 2, due to a change in solid angle of a factor of 4, isexpected if the intensity required to sustain the plasma at its boundaryis a constant. A decrease in plasma length (along the X-Axis) by afactor of 2 (decrease from 2 mm in FIG. 8A to 1 mm in FIG. 8B) resultedin an approximate doubling of the brightness of the radiation emitted bythe plasma for a specified laser beam input power because the powerabsorbed by the plasma is about the same, while the radiating area ofthe plasma was approximately halved (due to the decrease in length alongthe X-Axis). This experiment illustrated the ability to make the plasmasmaller by increasing the solid angle of the illumination from thelaser.

In general, larger solid angles of illumination can be achieved byincreasing the laser beam diameter and/or decreasing the focal length ofthe objective lens. If reflective optics are used for illumination ofthe plasma, them the solid angle of illumination can become much largerthan the experiment described above. For example, in some embodiments,the solid angle of illumination can be greater than about 2π (about6.28) steradians when the plasma is surrounded by a deep, curvedreflecting surface (e.g., a paraboloid or ellipsoid). Based on theconcept that a constant intensity of light is required to maintain theplasma at its surface, in one embodiment (using the same bulb and laserpower described in the experiment above) we calculated that a solidangle of 5 steradians would produce a plasma with its length equal toits diameter, producing a roughly spherical plasma.

FIG. 9 is a schematic diagram of a light source 900 for generatinglight. The light source 900 includes a sealed chamber 928 (e.g., asealed quartz bulb, sealed sapphire tube) that contains an ionizablemedium (not shown). The light source 900 also includes an energy source(not shown). The energy source provides energy to a region of thechamber 928 to produce a plasma 932. The plasma 932 generates and emitsa light 936 that originates from the plasma 932. The light 936 generatedby the light source 900 is directed through the walls 942 of the chamber928 toward the reflective surface 944 of the reflector 940. Thereflective surface 944 reflects the light generated by the light source900.

The walls 942 of the chamber 928 allow electromagnetic energy (e.g.,light) to pass through the walls 942. The refractive index of the wallsis a measure for how much the speed of the electromagnetic energy isreduced inside the walls 942. Properties (e.g., the direction ofpropagation) of the light ray 936 generated by the plasma 932 that isemitted through the walls 942 of the chamber 928 are modified due to therefractive index of the walls 942. If the walls 942 have a refractiveindex equal to that of the medium 975 internal to the chamber 928(typically near 1.0), the light ray 936 passes through the walls 942 aslight ray 936′. If, however, the walls have a refractive index greaterthan that of the internal medium 975, the light ray 936 passes throughthe walls as light ray 936″.

The direction of the light represented by light ray 936 is altered asthe light ray 936 enters the wall 942 having an index of refractiongreater than the medium 975. The light ray 936 is refracted such thatthe light ray 936 bends toward the normal to the wall 942. The lightsource 900 has a medium 980 external to the chamber 928. In thisembodiment, the medium 980 has an index of refraction equal to the indexof refraction of the medium 975 internal to the chamber 928. As thelight ray 936 passes out of the wall 942 into the medium 980 external tothe chamber 928, the light ray 936 is refracted such that the light ray(as light ray 936″) bends away from the normal to the wall 942 when itexits the wall 942 The light ray 936″ has been shifted to follow a routeparallel to the route the light ray 936′ would have followed had therefractive index of the wall 942 been equal to the refractive indices ofthe internal medium 975 and external medium 980.

This refractive shift of direction and the resulting position of thelight ray 936 (and 936′ and 936″) is described by Snell's Law ofRefraction:n₁ sin θ₁=n₂ sin θ₂  EQN. 1where, according to Snell's Law, n₁ is the index of refraction of themedium from which the light is coming, n₂ is the index of refraction ofthe medium into which the light is passing, θ₁ is the angle of incidence(relative to the normal) of the light approaching the boundary betweenthe medium from which the light is coming and the medium into which thelight is passing, and θ₂ is the angle of incidence (relative to thenormal) of the light departing from the boundary between the medium fromwhich the light is coming and the medium into which the light is passing(Hecht, Eugene, Optics, M. A., Addison-Wesley, 1998, p. 99-100,QC355.2.H42).

If the internal medium 975 does not have an index of refraction equal tothat of the external medium 980, the light ray 936 refracts to follow aroute according to Snell's Law. The route the light ray 936 follows willdiverge from the route that light ray 936′ follows when the internalmedium 975, wall 942, and external medium 980 do not have equal indicesof refraction.

If the refractive index of the walls 942 of the chamber 928 is equal tothe internal medium 975 and external medium 980, the reflective surface944 reflects light ray 936′ and produces a focused beam of light 956.If, however, the refractive index of the walls 942 is greater than theinternal medium 975 and external medium 980, the reflective surface 944reflects light ray 936″ and does not produce a focused beam (the lightray 936″ is dispersed producing light 960). Accordingly, it is thereforedesirable to have a light source that includes a chamber and areflective surface with a shape configured to compensate for the effectof the refractive index of the walls of the chamber.

In alternative embodiments, the reflective surface 940 is configured toproduce a collimated beam of light when the refractive index of thewalls 942 of the chamber 928 is equal to that of the internal medium 975and external medium 980. However, if the refractive index of the walls942 of the chamber 928 is greater than that of the internal medium 975and external medium 980, the reflective surface 940 would produce anon-collimated beam of light (the reflected light would be dispersed,similarly as described above).

In other embodiments, aspects of the invention are used to compensatefor the effect of the refractive index of the walls 942 of the chamber928 for laser energy directed in to the chamber 928. Laser energy isdirected toward the reflective surface 944 of the reflector 940. Thereflective surface 944 reflects the laser energy through the walls 942of the chamber 928 toward the plasma 932 in the chamber 928 (similarlyas described herein with respect to, for example, FIGS. 5 and 6). If thewalls 942 of the chamber 928 have a refractive index greater than thatof the internal 975 and external 980 media, the direction of the laserenergy is altered as the energy enters the walls 942. In theseembodiments, if the reflective surface 944 of the reflector 942 has ashape configured to compensate for the effect of the refractive index ofthe walls of the chamber, the laser energy entering the chamber 928 willnot diverge. Rather, the laser energy entering the chamber 928 will beproperly directed to the location of the plasma 932 in the chamber 928,similarly as described herein. In this manner, principles of theinvention can be applied to electromagnetic energy (e.g., laser energy)that is directed in to the chamber 928 and electromagnetic energy (e.g.,light) produced by the plasma 932 that is directed out of the chamber928.

FIG. 10A is a schematic block diagram of a light source 1000 a forgenerating light. The light source 1000 a includes a sealed chamber 1028a (e.g., a sealed quartz tube or sealed sapphire tube) that contains anionizable medium (not shown). The light source 1000 a also includes anenergy source 1015 a. In various embodiments, the energy source 1015 ais a microwave energy source, AC arc source, DC arc source, or RF energysource. The energy source 1015 a provides energy 1022 a to a region 1030a of the chamber 1028 a having the ionizable medium. The energy 1022 acreates a plasma 1032 a. The plasma 1032 a generates and emits a light1036 a that originates from the plasma 1032 a. The light source 1000 aalso includes a reflector 1040 a that has a reflective surface 1044 a.The reflective surface 1044 a of the reflector 1040 a has a shape thatis configured to compensate for the refractive index of the walls 1042 aof the chamber 1028 a.

The walls 1042 a of the chamber 1028 a are substantially transparent tothe light 1036 a (or to predefined wavelengths of electromagneticradiation in the light 1036 a). The light 1036 a (e.g., visible and/orultraviolet light) generated by the light source 1000 a is directedthrough the walls 1042 a of the chamber 1028 a toward the innerreflective surface 1044 a of the reflector 1040 a.

If the refractive index of the walls 1042 a is not equal to that of themedia internal and external (not shown) to the chamber 1028 a, theposition and direction of the light ray 1036 a is changed by passingthrough the walls 1042 a of the chamber 1028 a unless the reflectivesurface 1044 a has a shape that compensates for the refractive index ofthe walls 1042 a of the chamber 1028 a. The light 1036 a would disperseafter reflecting off the surface 1044 a of the reflector 1040 a.However, because the shape of the reflective surface 1044 a of thereflector 1040 a is configured to compensate for the refractive index ofthe walls 1042 a of the chamber 1028 a, the light 1036 a does notdisperse after reflecting off the surface 1044 a of the reflector 1040a. In this embodiment, the light 1036 a reflects off the surface 1044 aof the reflector 1040 a to produce a collimated beam of light.

FIG. 10B is a schematic block diagram of a light source 1000 b forgenerating light. The light source 1000 b includes a sealed chamber 1028b (e.g., a sealed quartz tube or sealed sapphire tube) that contains anionizable medium (not shown). The light source 1000 b also includes anenergy source 1015 b. The energy source 1015 b is electrically connectedto electrodes 1029 located in the chamber 1028 b. The energy source 1015b provides energy to the electrodes 1029 to generate an electricaldischarge in the chamber 1028 b (e.g., the region 1030 b of the chamber1028 b) to ignite the ionizable medium and produce and sustain a plasma1032 b. The plasma 1032 b generates and emits a light 1036 b thatoriginates from the plasma 1032 b. The light source 1000 b also includesa reflector 1040 b that has a reflective surface 1044 b. The reflectivesurface 1044 b of the reflector 1040 b has a shape that is configured tocompensate for the refractive index of the walls 1042 b of the chamber1028 b.

The walls 1042 b of the chamber 1028 b are substantially transparent tothe light 1036 b (or to predefined wavelengths of electromagneticradiation in the light 1036 b). The light 1036 b (e.g., visible and/orultraviolet light) generated by the light source 1000 b is directedthrough the walls 1042 b of the chamber 1028 b toward the innerreflective surface 1044 b of the reflector 1040 b.

If the refractive index of the walls 1042 b is not equal to that ofmedia internal and external (not shown) to the chamber 1028 b, thedirection and position of the light ray 1036 b is changed by passingthrough the walls 1042 b of the chamber 1028 b unless the reflectivesurface 1044 b has a shape that compensates for the refractive index ofthe walls 1042 b of the chamber 1028 b. The light 1036 b would disperseafter reflecting off the surface 1044 b of the reflector 1040 b.However, because the shape of the reflective surface 1044 b of thereflector 1040 b is configured to compensate for the refractive index ofthe walls 1042 b of the chamber 1028 b, the light 1036 b does notdisperse after reflecting off the surface 1044 b of the reflector 1040b. In this embodiment, the light 1036 b reflects off the surface 1044 bof the reflector 1040 b to produce a collimated beam of light.

FIG. 11 is a schematic block diagram of a light source 1100 forgenerating light, that embodies the invention. The light source 1100includes a sealed chamber 1128 (e.g., a sealed, cylindrical sapphirebulb) that contains an ionizable medium (not shown). The light source1100 provides energy to a region 1130 of the chamber 1128 having theionizable medium which creates a plasma 1132. The plasma 1132 generatesand emits a light 1136 (e.g., a high brightness light) that originatesfrom the plasma 1132. The light source 1100 also includes at least onelaser source 1104 that generates a laser beam that is provided to theplasma 1132 located in the chamber 1128 to initiate and/or sustain thehigh brightness light 1136.

In this embodiment, the laser source 1104 is a diode laser that outputsa laser beam 1120. The optical lens 1120 is configured to focus thelaser beam 1122 to produce a smaller diameter laser beam 1124. The laserbeam 1124 passes through an aperture or window 1172 located in the base1124 of a curved reflective surface 1140 and is directed toward thechamber 1128. The chamber 1128 is substantially transparent to the laserbeam 1124. The laser beam 1124 passes through the chamber 1128 andtoward the region 1130 of the chamber 1128 where the plasma 1132 exists(or where it is desirable for the plasma 1132 to be generated by thelaser 1124 and sustained).

In this embodiment, the ionizable medium is ignited by the laser beam1124. In alternative embodiments, the light source 1100 includes anignition source (e.g., a pair of electrodes or a source of ultravioletenergy) that, for example, generates an electrical discharge in thechamber 1128 (e.g., in the region 1130 of the chamber 1128) to ignitethe ionizable medium. The laser source 1104 then provides laser energyto the ionized medium to sustain the plasma 1132 which generates thelight 1136. The chamber 1128 is substantially transparent to the light1136 (or to predefined wavelengths of electromagnetic radiation in thelight 1136). The light 1136 (e.g., visible and/or ultraviolet light)generated by the light source 1100 is then directed out of the chamber1128 toward an inner surface 1144 of the reflective surface 1140.

The reflective surface 1144 of the reflector 1140 has a shape thatcompensates for the refractive index of the walls 1142 of the chamber1128. If the refractive index of the walls 1142 is not equal to that ofthe media internal and external (not shown) to the chamber 1128, thespeed of the light 1136 would be changed by passing through the walls1142 of the chamber 1128 if the reflective surface 1144 does not have ashape that compensates for the refractive index of the walls 1142 of thechamber 1128 (similarly as described above with respect to FIG. 9).

FIG. 12 is a cross-sectional view of a light source 1200 incorporatingprinciples of the present invention. The light source 1200 includes asealed cylindrical chamber 1228 that contains an ionizable medium. Thelight source 1200 also includes a reflector 1240. The chamber 1228protrudes through an opening 1272 in the reflector 1240. The lightsource 1200 includes a support element 1274 (e.g. a bracket orattachment mechanism) attached to the reflector 1240. The supportelement 1274 is also attached to aback end 1280 of the chamber 1228 andlocates the chamber 1228 relative to the reflector 1240. The lightsource 1200 includes electrodes 1229 a and 1229 b (collectively 1229)located in the chamber 1228 that ignite the ionizable medium to producea plasma 1232. The electrodes 1229 a and 1229 b are spaced apart fromeach other along the Y-Axis with the plasma 1232 located betweenopposing ends of the electrodes 1229.

The light source 1200 also includes an energy source that providesenergy to the plasma 1232 to sustain and/or generate a light 1236 (e.g.,a high brightness light) that is emitted from the plasma 1232. The light1236 is emitted through the walls 1242 of the chamber 1228 and directedtoward a reflective surface 1244 of a reflector 1240. The reflectivesurface 1244 reflects the light 1236.

In some embodiments, the electrodes 1229 also are the energy source thatprovides energy to the plasma 1232 sustain and/or generate the light1236. In some embodiments, the energy source is a laser external to thechamber 1228 which provides laser energy to sustain and/or generate thelight 1236 generated by the plasma 1232, similarly as described hereinwith respect to other embodiments of the invention. For example, in oneembodiment, the light source 1200 includes a laser source (e.g., thelaser source 104 of FIG. 1) and associated laser delivery components andoptical components that provides laser energy to the plasma 1232 toand/or generate the light 1236.

If the refractive index of the walls 1242 of the chamber is equal tothat of the media internal and external (not shown) to the chamber 1228,and the reflective surface 1244 of the reflector 1240 is a parabolicshape, the light 1236 reflected off the surface 1244 produces acollimated beam of light 1264. If the refractive index of the walls 1242of the chamber is equal to that of the media internal and external tothe chamber 1228, and the reflective surface 1244 of the reflector 1240is an ellipsoidal shape, the light 1236 reflected off the surface 1244produces a focused beam of light 1268.

If the refractive index of the walls 1242 of the chamber 1228 is greaterthan that of the media internal and external to the chamber 1228, thedirection and position of the light ray 1236 is changed by passingthrough the walls 1242 of the chamber 1228 unless the reflective surface1244 of the reflector 1240 has a shape that compensates for therefractive index of the walls 1242 of the chamber 1228. The light ray1236 would disperse after reflecting off the surface 1244 of thereflector 1240. However, because the shape of the reflective surface1244 of the reflector 1240 is configured to compensate for therefractive index of the walls 1242 of the chamber 1228, the light 1236does not disperse after reflecting off the surface 1244 of the reflector1240.

In this embodiment, the refractive index of the walls 1242 of thechamber 1228 is greater than that of the internal and external media andthe reflective surface 1242 has a modified parabolic shape to compensatefor the refractive index of the walls 1242. The modified parabolic shapeallows for the reflected light 1236 to produce the collimated beam oflight 1264. If a parabolic shape was used, the reflected light 1236would not be collimated, rather the reflected light would be dispersed.A modified parabolic shape means that the shape is not a pure parabolicshape. Rather, the shape has been modified sufficiently to compensatefor the aberrations that would otherwise be introduced into thereflected light 1236. In some embodiments, the shape of the reflectivesurface 1242 is produced to reduce the error (e.g., dispersing of thereflected light 1236) below a specified value.

In some embodiments, the shape of the reflective surface 1242 isexpressed as a mathematical equation. In some embodiments, by expressingthe shape of the reflective surface 1242 as a mathematical equation, itis easier to reproduce the shape during manufacturing. In someembodiments, parameters of the mathematical equation are selected toreduce error due to the refractive index of the walls 1242 of thechamber 1228 below a specified value.

The light source 1200 includes a seal assembly 1250 at the top of thechamber 1228. The light source 1200 also includes a fitting 1260 at thebottom end of the chamber 1228. The seal assembly 1250 seals the chamber1228 containing the ionizable medium. In some embodiments, the sealassembly 1250 is brazed to the top end of the chamber 1228. The sealassembly 1250 can include a plurality of metals united at hightemperatures. The seal assembly 1250 can be, for example, a valve stemseal assembly, a face seal assembly, an anchor seal assembly, or a shaftseal assembly. In some embodiments the seal assembly 1250 ismechanically fastened to the top end of the chamber 1228. In someembodiments, there are two seal assemblies 1250, located at the two endsof the chamber 1228.

The fitting 1260 allows for filling the chamber with, for example, theionizable medium or other fluids and gases (e.g., an inert gas tofacilitate ignition). The fitting 1260 also allows for controlling thepressure in the chamber 1228. For example, a source of pressurized gas(not shown) and/or a relief valve (not shown) can be coupled to thefitting to allow for controlling pressure in the chamber 1228. Thefitting 1260 can be a valve that allows the ionizable medium to flowinto the chamber 1228 through a gas inlet (not shown).

FIG. 13 is a graphical representation of a plot 1300 of the blur ordispersal produced by a reflective surface (e.g., the reflective surface1244 of FIG. 12) for a reflective surface having various shapes that areexpressed as a mathematical expression having the form of EQN. 2. Theblur or dispersal is the radius by which light rays reflected off of thereflective surface miss the desired remote focus point for the reflectedlight (e.g., the reflected light 1268 of FIG. 12 in the situation wherethe shape of the reflective surface 144 is a modified elliptical shape).

$\begin{matrix}{{r(z)} = \sqrt{\frac{{a_{1}z} + {a_{2}z^{2}} + {a_{3}z^{3}} + \ldots + {a_{n}z^{n}}}{1 + {b_{1}z} + {b_{2}z^{2}} + \ldots + {b_{m}z^{m}}}}} & {{EQN}.\mspace{14mu} 2}\end{matrix}$

The X-axis 1304 of the plot 1300 is the position along the optical axis(in millimeter units) where a particular ray of light reflects from thereflective surface (e.g. the reflective surface 1244) of FIG. 12). TheY-Axis 1308 of the plot 1300 is the radius (i.e., blur or dispersal) inmillimeter units. The cylindrical chamber has an outer diameter (alongthe X-axis) of 7.11 mm and an inner diameter of 4.06 mm. Curve 1312shows the radius by which light rays reflected off of the location alongthe optical axis of the reflective surface miss the desired remote focuspoint for the reflected light, in which the reflective surface isexpressed as a mathematical equation (EQN. 2) in which n=2 and m=0.Curve 1316 shows the radius by which light rays reflected off of thelocation along the optical axis of the reflective surface miss thedesired remote focus point for the reflected light, in which thereflective surface is expressed as a mathematical equation (EQN. 2) inwhich n=3 and m=1. Curve 1320 shows the radius by which light raysreflected off of the location along the optical axis of the reflectivesurface miss the desired remote focus point for the reflected light, inwhich the reflective surface is expressed as a mathematical equation(EQN. 2) in which n=4 and m=4. Curve 1324 shows the radius by whichlight rays reflected off of the location along the optical axis of thereflective surface miss the desired remote focus point for the reflectedlight, in which the reflective surface is expressed as a mathematicalequation (EQN. 2) in which n=5 and m=5.

In this embodiment, a ray tracing program was used to select (e.g.,optimize) the parameters of the mathematical equation so the shape ofthe reflective surface compensates for the refractive index of the wallsof the chamber containing the ionizable medium. Referring to FIG. 13 andEQN 2, the parameters are the order and coefficients of the mathematicalequation. In this embodiment, a ray tracing program was used todetermine the paths of the light rays emitted through the walls of achamber in which the walls had a refractive index greater than that ofthe media internal and external to the chamber, and reflected off areflective surface with a shape described according to EQN. 2 withselected order and coefficients. In this embodiment, the ray tracingprogram graphs the radii by which light rays originating at points alongthe optical path of the reflective surface miss the desired remote focuspoint.

In this embodiment, the order and coefficients of the rationalpolynomial (EQN. 2) are adjusted until the radii by which light raysmiss the remote focus point are within a threshold level of error. Inother embodiments, the order and/or coefficients are adjusted until thefull width at half maximum (FWHM) of the light rays emitted by theplasma converge within a specified radius of the remote focus point. Inone embodiment, the specified radius is 25 μm.

In other embodiments, the ray tracing program graphs the radii by whichlight rays originating at points along the optical path of thereflective surface miss a target collimated area at a specified distancefrom the vertex of the reflective surface. The parameters of themathematical equation expressing the shape of the reflective surface areadjusted until the radii by which light rays miss the target collimatedarea are within a threshold level of error. In other embodiments, theorder and/or coefficients are adjusted until the full width at halfmaximum (FWHM) of the light rays emitted by the plasma is located withina specified radii of a target collimated area at a specified distancefrom the vertex of the reflective surface.

In alternative embodiments of the invention, alternative forms ofmathematical equations can be used to describe or express the shape ofthe reflective surface of the reflector (e.g., reflective surface 1244of reflector 1240 of FIG. 12). The principles of the present inventionare equally applicable to light sources that have different chambershapes and/or reflective surface shapes. For example, in someembodiments, the reflective surface of the reflector has a shape that isa modified parabolic, elliptical, spherical or aspherical shape that isused to compensate for the refractive index of the walls of the chamber.

FIG. 14 is a schematic block diagram of a portion of a light source1400, according to an illustrative embodiment of the invention. Thelight source 1400 includes a sealed chamber 1428 that includes anionizable medium. The light source 1400 also includes a first reflector1440 that has a reflective surface 1444. The reflective surface 1444 canhave, for example, a parabolic shape, elliptical shape, curved shape,spherical shape or aspherical shape. In this embodiment, the lightsource 1400 has an ignition source (not shown) that ignites an ionizablegas (e.g., mercury or xenon) in a region 1430 within the chamber 1428 toproduce a plasma 1432.

In some embodiments, the reflective surface 1444 can be a reflectiveinner or outer surface. In some embodiments, a coating or film islocated on the inside or outside of the chamber to produce thereflective surface 1444.

A laser source (not shown) provides a laser beam 1416 that is directedtoward a surface 1424 of a second reflector 1412. The second reflector1412 reflects the laser beam 1420 toward the reflective surface 1444 ofthe first reflector 1440. The reflective surface 1444 reflects the laserbeam 1420 and directs the laser beam toward the plasma 1432. Therefractive index of the walls 1442 of the chamber 1430 affects the laserbeam 1416 as it passes through the walls 1442 in to the chamber 1430similarly as light passing through the walls 1442 of the chamber 1430 isaffected as described previously herein. If the shape of the reflectivesurface 1444 is not selected to compensate for the refractive index, thelaser energy disperses or fails to focus after entering the chamber 1430and is not focused on the plasma 1432. Accordingly, in this embodiment,the reflective surface 1444 of the reflector has a shape that isselected to compensate for the refractive index of the walls 1442 of thechamber 1430 (similarly as described previously herein with respect to,for example, FIGS. 12 and 13).

The laser beam 1416 provides energy to the plasma 1432 to sustain and/orgenerate a high brightness light 1436 that is emitted from the plasma1432 in the region 1430 of the chamber 1428. The high brightness light1436 emitted by the plasma 1432 is directed toward the reflectivesurface 1444 of the first reflector 1440. At least a portion of the highbrightness light 1436 is reflected by the reflective surface 1444 of thefirst reflector 1440 and directed toward the second reflector 1412.Because the reflective surface 1444 of the reflector has a shape that isselected to compensate for the refractive index of the walls 1442 of thechamber 1430, the light 1436 reflected by the reflective surface 1444produces the desired collimated beam of light 1436 that is directedtowards the second reflector 1412.

The second reflector 1412 is substantially transparent to the highbrightness light 1436 (e.g., at least one or more wavelengths ofultraviolet light). In this manner, the high brightness light 1436passes through the second reflector 1412 and is directed to, forexample, a metrology tool (not shown). In some embodiments, the light1436 is directed to a tool used for photoresist exposure, conductingellipsometry (e.g., UV or visible), thin film measurements.

In some embodiments, the high brightness light 1436 is first directedtowards or through a window or additional optical elements before it isdirected to a tool.

FIG. 15A is a cross-sectional view of a light source 1500 incorporatingprinciples of the present invention. FIG. 15B is a sectional view (inthe Y-Z plane) of the light source 1500 of FIG. 15A. The light source1500 includes a housing 1510 that houses various elements of the lightsource 1500. The housing 1510 includes a sealed chamber 1522 and has anoutput 1580 which includes an optical element 1520 (e.g., a quartzdisk-shaped element) through which light can exit the housing 1510. Thelight source 1500 includes a sealed chamber 1528 that contains anionizable medium (not shown). The light source 1500 also includes areflector 1540. The light source 1500 also includes a blocker 1550. Thelight source 1500 includes electrodes 1529 a and 1529 b (collectively1529) located in part in the chamber 1528 that ignite the ionizablemedium to produce a plasma (not shown). The electrodes 1529 a and 1529 bare spaced apart from each other (along the Y-Axis) with the plasmalocated between opposing ends of the electrodes 1529.

In some embodiments, the electrodes 1529 also are the energy source thatprovides energy to the plasma to sustain and/or generate the light. Inthis embodiment, the energy source is a laser (not shown) external tothe chamber 1528 which provides laser energy 1524 (e.g., infrared light)to sustain and/or generate the light 1530 (e.g., a high brightness lightincluding ultraviolet and/or visible wavelengths) generated by theplasma, similarly as described herein with respect to other embodimentsof the invention. The laser energy 1524 enters the chamber 1528 on afirst side 1594 of the chamber 1528. In some embodiments, the lightsource 1500 also includes associated laser delivery components andoptical components that provide laser energy to the plasma to sustainand/or generate the light 1530. In this embodiment, the light source1500 includes an optical element 1560 to delivery the laser energy 1524from the laser to the plasma to sustain and/or generate the light 1530that is emitted from the plasma.

The light 1530 is emitted through the walls of the chamber 1528. Some ofthe light 1530 emitted through the walls of the chamber 1528 propagatestoward a reflective surface 1532 of the reflector 1540. The reflectivesurface 1532 reflects the light through the optical element 1520 in thehousing 1510 to a focal point 1525 of the reflector 1540. Some of thelight 1536 propagates toward the optical element 1560. The opticalelement 1560 absorbs the light 1536, and the light 1536 is not reflectedthrough the optical element 1520. As a result, the light reflected tothe focal point 1525 is the light 1530 emitted from the plasma that isreflected by the reflector 1540 along paths shown as the regions 1540and 1541. Consequently, the light source 1500 includes dark region 1542due to the light that is radiated toward the optical element 1560 andtherefore not reflected to the focal point 1525 of the reflectors 1540.

Some of the laser energy delivered to the plasma is not absorbed by theplasma. The laser energy that is not absorbed (laser energy 1556)continues to propagate along the positive X-Axis direction towards theend of the housing 1510. The blocker 1550 is suspended on a second side1596 of the chamber 1528. The blocker 1550 is suspended along a path1562 the laser energy 1556 travels. The blocker 1550 is coupled to anarm 1555 that suspends the blocker 1550 in the chamber 1522 of the lightsource 1500. The blocker 1550 blocks the laser energy 1556 to prevent itfrom propagating toward the end of the housing and through an output1580 of the light source 1500.

In this embodiment, the blocker 1550 is a mirror that deflects the laserenergy 1556 that is not absorbed by the plasma away from the opening1520 and towards the walls of the housing 1510 (illustrated as laserenergy 1584). The blocker 1550 reflects the laser energy 1556 toward awall 1588 of the housing 1510. The housing 1510 absorbs part of thereflected laser energy 1584 and reflects part of the laser energy 1584toward the opposite wall 1592 of the housing 1510. A portion of thelaser energy 1584 is absorbed each time it impacts a wall (e.g., wall1588 or 1592) of the housing 1510. Repetitive impact of the laser energy1584 with the walls of the housing 1510 causes the laser energy 1584 tobe substantially (or entirely) absorbed by the walls of the housing1510. The blocker 1550 prevents laser energy (e.g., infrared wavelengthsof electromagnetic energy) from exiting the housing 1510 through theopening 1580 by deflecting the laser energy 1556 using the blocker 1550.As a result, only the light produced by the plasma (e.g., ultravioletand/or visible wavelengths) exits the housing 1510 through the opening1580.

The blocker 1550 is suspended in the housing 1510 in a location wherethe blocker 1550 would not deflect light 1530 reflected by the reflector1540 through the opening 1580 to the focal point 1525. The blocker 1550does not deflect the light 1530 because the blocker 1550 is located inthe dark region 1542. In addition, the arm 1555 coupled to the blocker1550 also does not deflect the light 1530 because the arm is positionedin the housing 1510 in a location that is aligned with the electrode1529 a along the positive X-Axis direction relative to the electrode1529 a. In this manner, the blocker 1550 and arm 1555 are positioned tominimize their blocking of the light 1530.

The dark region 1542 tapers as the region 1542 approaches the opening1580. To prevent the blocker 1550 from deflecting light reflected by thereflector 1540, the laser energy blocker 1550 is positioned at alocation along the X-Axis where the cross-sectional area (in the Y-Zplane) of the blocker 1550 is equal to or less than the cross sectionalarea (in the Y-Z plane) of the dark region 1542. As a result, thesmaller the cross-sectional area (in the Y-Z plane) of the blocker 1550,the closer along the X-Axis the blocker 1550 can be placed to theopening 1580.

In some embodiments, the laser energy blocker 1550 is made of anymaterial that reflects the laser energy 1556. In some embodiments, theblocker 1550 is configured to reflect the laser energy 1556 back towardthe ionized medium in the chamber 1528. In some embodiments, the blocker1550 is a coating on a portion of the chamber 1528. In some embodiments,the blocker is a coating on the optical element 1520 at the opening1580.

In some embodiments, the laser energy blocker 1550 is made of a materialthat absorbs, rather than reflects, the laser energy 1556 (e.g.,graphite). In some embodiments in which the blocker absorbs the laserenergy 1556, the blocker 1550 heats up because it absorbs the laserenergy 1556.

In some embodiments, the blocker 1550 is cooled. The blocker 1550 caninclude one or more coolant channels in the blocker 1550. The lightsource can also include a coolant supply coupled to the coolant channelwhich provides coolant to the coolant channel to cool the blocker 1550.In some embodiments, the light source 1500 includes a gas source (e.g.,a pressurized gas canister or gas blower) to blow gas (e.g., air,nitrogen, or any other gas) on the blocker 1550 to cool the blocker1550. In some embodiments, the light source 1500 includes one or moretubes (e.g., copper tubes) that wind around the laser energy blocker1550. The light source 1500 flows a coolant (e.g., water) through thetubes to cool the blocker 1550.

By way of illustration, an experiment was conducted to generateultraviolet light using a light source, according to an illustrativeembodiment of the invention. A specially constructed quartz bulb with avolume of 1 cm³ was used as the chamber of the light source (e.g., thechamber 128 of the light source 100 of FIG. 1) for experiments usingxenon as the ionizable medium in the chamber. The bulb was constructedso that the chamber formed within the quartz bulb was in communicationwith a pressure controlled source of xenon gas. FIG. 16 is a graphicalrepresentation of brightness as a function of the pressure in a chamberof a light source, using a light source according to the invention. FIG.16 illustrates a plot 1600 of the brightness of a high brightness lightproduced by a plasma located in the chamber as a function of thepressure in the chamber.

The laser source used in the experiment was a 1.09 micron, 200 watt CWlaser and it was focused with a numerical aperture of 0.25. Theresulting plasma shape was typically an ellipsoid of 0.17 mm diameterand 0.22 mm length. The Y-Axis 1612 of the plot 1600 is the brightnessin watts/mm² steradian (sr). The X-Axis 1616 of the plot 1600 is thefill pressure of Xenon in the chamber. Curve 1604 is the brightness ofthe high brightness light (between about 260 and about 400 nm) producedby a plasma that was generated. Curve 1608 is the brightness of the highbrightness light (between about 260 and about 390 nm) produced by theplasma. For both curves (1604 and 1608), the brightness of the lightincreased with increasing fill temperatures. Curve 1604 shows abrightness of about 1 watts/mm² sr at about 11 atmospheres whichincreased to about 8 watts/mm² sr at about 51 atmospheres. Curve 1608shows a brightness of about 1 watts/mm² sr at about 11 atmospheres whichincreased to about 7.4 watts/mm² sr at about 51 atmospheres. Anadvantage of operating the light source with increasing pressures isthat a higher brightness light can be produced with higher chamber fillpressures.

Variations, modifications, and other implementations of what isdescribed herein will occur to those of ordinary skill in the artwithout departing from the spirit and the scope of the invention asclaimed. Accordingly, the invention is to be defined not by thepreceding illustrative description but instead by the spirit and scopeof the following claims.

1. A light source, comprising: a chamber; an energy source for providingenergy to a gas within the chamber to produce a plasma that generates alight emitted through the walls of the chamber; and a reflector thatreflects the light emitted through the walls of the chamber, thereflector comprising a reflective surface with a shape configured tocompensate for the refractive index of the walls of the chamber.
 2. Thelight source of claim 1, wherein the energy source is at least one laserexternal to the chamber.
 3. The light source of claim 1, wherein theenergy source is also an ignition source within the chamber.
 4. Thelight source of claim 1, wherein the energy source is at least one of amicrowave energy source, AC arc source, DC arc source, laser or RFenergy source.
 5. The light source of claim 1, wherein the chamber is asealed chamber.
 6. The light source of claim 1, wherein the chambercomprises a cylindrical tube.
 7. The light source of claim 6, whereinthe cylindrical tube is tapered.
 8. The light source of claim 6, whereinthe chamber comprises one or more seals at one or both ends of thecylindrical tube.
 9. The light source of claim 1, wherein the chambercomprises sapphire, quartz, fused quartz, Suprasil quartz, fused silica,Suprasil fused silica, MgF₂, diamond, single crystal quartz, or CaF₂.10. The light source of claim 1, wherein the chamber comprises adielectric material.
 11. The light source of claim 1, wherein thechamber comprises an ultraviolet transparent dielectric material. 12.The light source of claim 1, wherein the shape comprises a modifiedparabolic shape, elliptical shape, spherical shape or aspherical shape.13. The light source of claim 1, further comprising an ignition sourcefor ionizing a gas within the chamber.
 14. The light source of claim 13,wherein the ignition source comprises electrodes, an ultravioletignition source, a capacitive ignition source, an inductive ignitionsource, a flash lamp, a pulsed laser, or a pulsed lamp.
 15. The lightsource of claim 13, wherein the ignition source comprises electrodeslocated on opposite sides of the plasma.
 16. The light source of claim1, wherein the chamber protrudes through an opening in the reflector.17. The light source of claim 1, further comprising a support elementthat locates the chamber relative to the reflector.
 18. The light sourceof 17, wherein the support element comprises a fitting to allow at leastone of pressure control or filling of the chamber.
 19. The light sourceof claim 1, further comprising at least one optical element formodifying a property of the high brightness light emitted through thewalls of the chamber and reflected by the reflector.
 20. The lightsource of claim 19, wherein the optical element is a mirror or a lens.21. The light source of claim 19, wherein the optical element isconfigured to deliver the high brightness light emitted through thewalls of the chamber and reflected by the reflector to a tool.
 22. Thelight source of claim 21, wherein the tool comprises a wafer inspectiontool, a microscope, an ultraviolet microscope, a reticle inspectionsystem, a metrology tool, a lithography tool, or an endoscopic tool. 23.The light source of claim 2, wherein the laser is a pulse laser, acontinuous-wave fiber laser, or a diode laser.
 24. A method forproducing light, comprising: emitting a light through the walls of achamber; and using a reflective surface of a reflector to reflect thelight, wherein the reflective surface has a shape configured tocompensate for the refractive index of the walls of the chamber.
 25. Themethod of claim 24, comprising flowing gas into the chamber.
 26. Themethod of claim 24, comprising igniting the gas in the chamber toproduce an ionized gas.
 27. The method of claim 26, comprising directingenergy to the ionized gas to produce a plasma that generates the light.28. The method of claim 26, comprising directing laser energy into thechamber from at least one laser external to the chamber.
 29. The methodof claim 28, comprising directing the laser energy through an opticalelement that modifies a property of the laser energy.
 30. The method ofclaim 24, comprising directing the reflected light through an opticalelement to modify a property of the reflected light.
 31. The method ofclaim 30, comprising directing the reflected light to a tool.
 32. Themethod of claim 23, comprising controlling the pressure of the chamber.33. The method of claim 24, wherein the shape is expressed as amathematical equation.
 34. The method of claim 33, wherein parameters ofthe equation are selected to reduce error due to the refractive index ofthe walls of the chamber below a specified value.
 35. The method ofclaim 24, wherein the reflective surface has a shape configured tocompensate for the refractive index of the walls of the chamber.
 36. Themethod of claim 35, wherein the reflective surface produces a collimatedor focused beam of reflected light.
 37. The method of claim 24 whereinthe reflective surface has modified parabolic, elliptical, spherical oraspherical shape to compensate for the refractive index of the walls ofthe chamber producing a focused, reflected high brightness light.
 38. Alight source, comprising: a chamber; an laser source for providingelectromagnetic energy to a gas within the chamber to produce a plasmathat generates a light emitted through the walls of the chamber; and areflector that reflects the electromagnetic energy through the walls ofthe chamber and the light emitted through the walls of the chamber, thereflector comprising a reflective surface with a shape configured tocompensate for the refractive index of the walls of the chamber.
 39. Alight source, comprising: a chamber; a means for providing energy to agas within the chamber to produce a plasma that generates a lightemitted through the walls of the chamber; and a means for reflecting thelight emitted through the walls of the chamber, the reflecting meanscomprising a reflective surface with a shape configured to compensatefor the refractive index of the walls of the chamber.